calculator.svelte.ts: back recompute-on-access getters with $derived

[grzanka]

Part of the code-health / complexity review of src/. Correctness/perf smell
in the hottest state module.

Summary

In src/lib/state/calculator.svelte.ts the public getters that templates read
on every render — rows, validationSummary, and stpDisplayUnit (around
lines 377–397) — each call a compute*() function on every property access
rather than exposing memoized $derived values.

get rows() { return computeRows(); }                 // re-parses every row, every read
get stpDisplayUnit() { return getStpDisplayUnit(); } // re-resolves unit, every read
get validationSummary() { return computeValidationSummary(); }

Why this needs doing

• computeRows() iterates all input rows and, per row, re-runs parseRow()
  (energy parsing + unit conversion + result lookup) and resolveParticleMass().
  These getters are read multiple times per render (table body, footer,
  validation banner, export wiring), so the same parse work runs several times
  for a single frame.
• It side-steps Svelte 5's fine-grained reactivity. $derived memoizes and only
  recomputes when a tracked dependency actually changes; a plain getter cannot,
  so we pay full cost on every read regardless of whether inputs changed.
• It's a latent correctness hazard: any code that reads the getter twice and
  assumes a stable reference (e.g. === identity checks, or capturing then
  comparing) gets a fresh object each time.

The project mandates runes; this is exactly the case $derived exists for.

Scope

• Introduce module-level $derived / $derived.by values for the three
  computations (and any sibling getter in the file with the same pattern — audit
  the whole returned object).
• Keep the public getter surface identical (get rows() returns the derived
  value) so no call sites change.
• Confirm the compute* functions are pure reads of reactive state (no
  side-effects); if any mutate state, that must be untangled first.

Acceptance criteria

• rows, validationSummary, stpDisplayUnit are backed by $derived.
• Reading a getter twice without an input change returns the same
  memoized value (add a focused unit test asserting referential stability).
• No behavioural change in calculator output; existing
  calculator-state.test.ts / calculator-url.test.ts pass unchanged.
• pnpm check, pnpm lint, pnpm test pass.

Files

• src/lib/state/calculator.svelte.ts
• src/tests/unit/calculator-state.test.ts (add memoization assertion)

AI logging

• CHANGELOG-AI.md entry + short log.